Users can use a variety of devices, such as PCs, tablets, mobile phones, and other computing devices to consume and/or interact with online content. Further, users can use separate browsers or applications on the same device or different devices to consume and/or interact with online content. Each device, browser, and/or application can have its own identifier. For example, IOS devices can utilize an Identifier for Advertisers (“IDFA”) or ANDROID devices can utilize an Advertising ID. Browsers can utilize other identifiers, many of which can be stored in cookies associated with separate domains. Ad systems, third-party data aggregators, merchants, and publishers can collect activity data based on a device's interactions with websites and other online content. Because each device, browser, and/or application can have its own identifier, identifying all devices, browsers, and/or applications used by a particular user and the associated activity data can be difficult. Further, some parties, such as merchants and publishers, may not have access to all of the activity data (e.g., activity data related to content that they do not own). Accordingly, they may not be able to determine which devices, browsers, and/or applications are used by the same user.